According to the Horizon Hobbies site the umx timber can run off a 200–280mAh LiPo. If your only increasing the battery from a 280mAh to a 300mAH you should be fine in terms of flying. Just be prepared to adjust the CG a bit and use as bit more throttle in flight
Not familiar with the turbo, but I fly my old UMX timber with bigger batteries a lot. I use 400mAh all the time but I had to glue a little balsa plate in the fuselage so the battery could be moved farther back. I can get 15 min on that battery if I fly easy.